1. Field of the Invention
This invention relates generally to a method and apparatus for continuously casting molten aluminum into a plate-shaped bearing alloy by belt casting means, and more particularly to such a casting method and apparatus in which crystals can be prevented from being coarsened.
2. Description of the Prior Art
As an apparatus for continuously casting a metal which melts at a relatively low temperature, for example, aluminum, aluminum alloys, zinc, etc., a belt casting machine has been known in which casting is carried out between a pair of endless belts. In the known belt casting machine, each belt is passed over or around a plurality of rollers. A horizontal or slightly inclined casting space is defined between substantially horizontal portions of the belts. The belts are driven by driving rollers so as to travel while being cooled by a cooling system. A molten metal is supplied into the casting space to be cooled by the belts, thereby solidifying into the shape of a plate. Plate-shaped materials are continuously fed out of the casting space. The aforesaid belt casting machine of the movable casting mold type is superior in a casting speed and accordingly in the productivity to a continuous casting machine of the fixed casting mold type.
A bearing lined with an aluminum bearing alloy, which is referred to as xe2x80x9caluminum alloy bearing,xe2x80x9d is generally used for engines of automobiles or industrial machines. The aluminum alloy bearing is manufactured sequentially through steps of casting, rolling, cladding, heat treatment and machining. More specifically, molten metals are cast into the shape of a plate. The cast plate is rolled in the rolling step. The cast plate is then cladded on a steel sheet so that a bimetal is made. The bimetal is annealed so that the bonding strength between the cast plate and the steel sheet is improved. Thereafter, the bimetal is machined to be finally formed into a semi-cylindrical or cylindrical bearing.
Manufacturers of engine bearings have used the aforesaid belt casting machine for continuously casting the aluminum alloy into the cast plate to thereby improve the productivity. In the belt casting machine, however, a cooling rate for the cast plate is low since the casting speed is high. Thus, the belt casting machine assumes that a slow cooling state is obtained. As a result, coarsening and segregation of crystals such as crystallized Sn and Si are easy to occur in the aluminum alloy containing Sn, Si, etc. Further, in an aluminum alloy containing various elements for improvement of the bearing characteristic, too, crystals of the intermetallic compound are easy to be coarsened and to be segregated. The plasticity of the alloy is reduced when the crystals are segregated or coarsened in the aluminum alloy. As a result, cracks occur in the alloy in the subsequent rolling and cladding steps where plastic deformation processing is executed. Further, decrease in the fatigue strength and wear resistance of the alloy as the bearing characteristics reduces the effect of addition of various elements for the improvement of the bearing characteristics.
Therefore, it is an object of the present invention is to provide a method and apparatus for continuous casting of aluminum bearing alloy in which coarsening and segregation of the crystals can be prevented when the aluminum bearing alloy is continuously cast into the shape of a plate.
According to a first feature of the invention, it is provided a method of continuous casting of a molten aluminum bearing alloy in which a casting space is defined between substantially parallel opposed portions of a pair of travelling endless belts and the molten aluminum bearing alloy is supplied into the casting space to be continuously cast into the shape of a plate. The method comprises the step of controlling a cooling rate xcex94T during solidification of the aluminum bearing alloy so that the cooling rate xcex94T ranges between 3 and 6xc2x0 C./sec. where xcex94T=(Txe2x88x92500)/t, T is a temperature when the casting of the aluminum bearing alloy starts, and t is a cooling time in sec. between start of casting and the time when the temperature of the aluminum bearing alloy decreases to 500xc2x0 C.
According to the above-described method, the cooling rate of 3 to 6xc2x0 C./sec. is higher than one of the conventional belt casting machine, that is, 1 to 2xc2x0 C./sec. When the aluminum alloy is solidified at such a high cooling rate, the crystal is not coarsened nor segregated. Further, occurrence of cracks can be prevented in the subsequent rolling and cladding. Additionally, the bearing characteristics can be prevented from being reduced.
In a second feature, the aluminum bearing alloy comprises, by mass, 3 to 40% Sn, 0.5 to 7% Si, 0.05 to 2% Fe, and the balance of Al and unavoidable impurities, and a ternary intermetallic compound of Alxe2x80x94SIxe2x80x94Fe is crystallized. In a third feature, the aluminum bearing alloy comprises, by mass, 3 to 40% Sn, 0.5 to 7% Si and 0.05 to 2% Fe and at least one or more of 0.01 to 3% each of Mn, V, Mo, Cr, Co, Ni and W, and the balance of Al and unavoidable impurities and a multi-element intermetallic compound of Alxe2x80x94SIxe2x80x94Fe containing said at least one or more of Mn, V, Mo, Cr, Co, Ni and W is crystallized.
In a fourth feature, the aluminum bearing alloy comprises at least one or more of 0.01 to 2% each of B, Ti and Zr. In a fifth feature, the aluminum bearing alloy comprises at least one or more of 0.1 to 5% each of Cu, Mg and Zn.
The technical background of the development of the aforesaid novel aluminum alloy will now be described. With recent development of high performance engines, engine bearings necessitate further improvement in the fatigue strength and wear resistance. Regarding the fatigue strength, elements such as Cu, Mn and V are added to the aluminum alloy to strengthen the latter. For the purpose of improvement in the wear resistance, JP-A-58-64332 discloses that Si is added to the aluminum alloy and the size and distribution of Si particles crystallized in the aluminum alloy are controlled. Further, JP-A-58-67841 discloses that Mn, Fe, Mo, Ni, etc. are added to the aluminum alloy so that an intermetallic compound between Mn etc. and Al is crystallized in the aluminum alloy. These two cases propose an improvement in the conformability and anti-seizure property of the aluminum alloy, thereby improving the wear resistance.
The above-noted JP-A-58-64332 and JP-A-58-67841 disclose that a desired effect can be achieved when the sizes of Si particles and the intermetallic compound range between 5 xcexcm and 40 xcexcm, respectively. Generally, hard particles contained in Al are uniformly distributed to be used for strengthening the aluminum alloy, and the effect is larger as the size of particles becomes small. In the aforesaid two cases, however, when the size of Si and the intermetallic compound is controlled so as to range between 5 xcexcm and 40 xcexcm, the strength of the Al matrix and accordingly the fatigue strength of the Al alloy are reduced as the size of Si and the intermetallic compound is relatively large. Thus, the anti-seizure property cannot be improved when crystallized particles are rendered small for improvement in the fatigue strength. On the other hand, the fatigue strength cannot be improved when the crystallized particles are rendered large for improvement in the anti-seizure property and accordingly in the wear resistance.
The inventors developed an Al alloy by crystallizing a ternary intermetallic compound of Alxe2x80x94SIxe2x80x94Fe or a multi-element intermetallic compound containing Alxe2x80x94SIxe2x80x94Fe as the base. The Al alloy can improve the anti-seizure property and wear resistance without reduction in the fatigue strength. The ternary intermetallic compound of Alxe2x80x94SIxe2x80x94Fe and the multi-element intermetallic compound containing Alxe2x80x94SIxe2x80x94Fe as the base are exceedingly stable, and its basic shape is not changed even by the heat treatment after cladding with a back metal. More specifically, Si crystallizes as a eutectic in the form like a three-dimensionally connected coral. The crystallized Si is crushed to pieces by rolling after casting or rolling in the cladding with the back metal. Further, Si also changes its form by a subsequent heat treatment. This is a characteristic of Si and particularly in the heat treatment in which the temperature exceeds 300xc2x0 C., Si changes into a relatively rounded so that a surface tension thereof is reduced. This tendency is enhanced in a material containing a large amount of Sn, for example, an Alxe2x80x94Sn bearing alloy.
However, the aforesaid ternary intermetallic compound or multi-element intermetallic compound does not change its crystallized form (an example is shown in FIG. 3) and does not change its form at a temperature for a usual heat treatment. Further, the ternary or multi-element intermetallic compound is crushed in the rolling step with plastic deformation or the cladding step during manufacture of the bearing. However, as the result of crush, the intermetallic compound takes a form with a sharp edge such as a broken piece of an edged tool. FIG. 4 shows an example of such a form. Although Si particles are rounded and broken into pieces through the steps of rolling and heat treatment, the aforesaid ternary or multi-element intermetallic compound retains an aggressive form with a sharp edge.
The ternary or multi-element intermetallic compound has a lapping effect on a shaft even when its amount is small. Particularly, the ternary or multi-element intermetallic compound stabilizes the relationship between a shaft with an unstable initial operation and the bearing. Thus, the ternary or multi-element intermetallic compound is effective in improving the conformability. More concretely, the ternary or multi-element intermetallic compound scrapes off protrusions on the surface of the shaft and an edge such as burrs around nodular graphite on the surface of the shaft. The ternary or multi-element intermetallic compound further prevents the Al alloy from wear due to adhesion to the shaft, which is a disadvantage of the Al alloy. Additionally, the ternary or multi-element intermetallic compound further scrapes away an adherent matter to thereby prevent seizure due to the adherent matter. Moreover, the ternary or multi-element intermetallic compound is relatively large even after the rolling step. Minutely pulverized Si particles are distributed in the Al matrix, thereby improving the strength of the Al matrix. Consequently, both improvement in the wear resistance and anti-seizure property and improvement in the fatigue strength can be achieved.
The Al alloy is solidified with a cooling rate ranging between 3 and 6xc2x0 C./sec in accordance with the casting method of the present invention when the Al alloy crystallizing the ternary or multi-element intermetallic compound is cast. Consequently, the intermetallic compound can be controlled so that a crystalline size ranges between 40 and 55 xcexcm without coarsening the intermetallic compound. Further, a crystallized Si can also be controlled so as to be at or below 40 xcexcm. Thereafter, when the casting plate is rolled or when the back metal is cladded on the cast plate, the intermetallic compound is pulverized into a size ranging between 1 and 20 xcexcm and the crystallized Si is at or below 5 xcexcm.
The reasons for the amount limitation of each aforesaid component will be described below.
(1) Sn (3 to 40 Mass %)
Sn improves surface properties such as anti-seizure property, conformability and embeddability as a bearing. When the Sn content is less than 3%, the above-mentioned effects are small. When it exceeds 40%, mechanical properties of the bearing alloy are deteriorated with the result of reduction in the bearing performance. A preferable Sn content ranges between 6 and 20%.
(2) Si (0.5 to 7 Mass %)
Si dissolves in the aluminum matrix and partially crystallizes as a single substance of silicon particle to disperse finely, so as to enhance the fatigue strength of the material and serve to improve the anti-seizure property and wear resistance. On the other hand, Si is an essential element in order to form the Alxe2x80x94SIxe2x80x94Fe intermetallic compound and improves the lapping, anti-seizure property, and wear resistance. When the Si content is less than 0.5%, Si dissolves into the Al matrix such that the above effects are small. When it exceeds 7%, its crystal is coarsened, so as to reduce the fatigue strength of the bearing alloy. A preferable Si content ranges between 2 and 6%.
(3) Fe (0.05 to 2 Mass %)
Fe crystallizes mainly as the Alxe2x80x94SIxe2x80x94Fe intermetallic compound, so as to produce the above-described effects. The intermetallic compound containing Fe prevents seizure with a shaft and improves the wear resistance. The characteristic is effective when the Fe content ranges between 0.05 and 2%. When the Fe content is less than 0.05%, the above-mentioned effects are small. When the Fe content exceeds 2%, the compound is coarsened and the bearing alloy becomes brittle, whereupon the rolling work causes trouble. A preferable Fe content ranges between 0.07 and 1%.
(4) Mn, V, Mo, Cr, Co, Ni, and W (at Least One Element of These: 0.01 to 3 Mass % in a Total Amount)
These are optional elements which constitute the multi-element intermetallic compound in the present invention. More specifically, when a selected element a is added to Alxe2x80x94SIxe2x80x94Fe, a multi-element intermetallic compound of Alxe2x80x94SIxe2x80x94Fe-xcex1 is produced. The selected element dissolves in the aluminum matrix as a single substance to thereby strengthen the matrix. Effects of the multi-element intermetallic compound cannot be expected when the content of each element is less than 0.01%. When the content of each element exceeds 3%, the multi-element intermetallic compound is excessively coarsened such that the physical properties of the bearing alloy are degraded and plastic workability of the bearing alloy such as rolling is degraded. A preferable content ranges between 0.2 and 2%.
(5) B, Ti and Zr (at Least One of These Elements: 0.01 to 2 Mass % in a Total Amount)
These optional elements do not contribute to formation of the Alxe2x80x94SIxe2x80x94Fe intermetallic compound, dissolving in the aluminum matrix, so as to improve the fatigue strength of the bearing alloy. The aforesaid effect is small when the content is less than 0.01%. When the content exceeds 2%, the bearing alloy becomes brittle. A preferable content ranges between 0.02 and 0.5%.
(6) Cu, Mg, and Zn (at Least One of These: 0.1 to 5 Mass % in a Total Amount)
These optional elements are additional elements which improve the strength of the aluminum matrix. A solution treatment forces these elements to dissolve in the aluminum matrix. When these elements are cooled and aged, fine compounds can be precipitated. The effects cannot be expected when the additive amount is less than 0.1%. When the additive amount exceeds 5%, the compound becomes coarse. A preferable additive amount ranges between 0.5 and 4%.
The present invention also provides an apparatus for continuously casting a molten aluminum bearing alloy into the shape of a plate, comprising belt casting means including a pair of travelling endless belts and a casting space defined between substantially parallel opposed portions of a pair of travelling endless belts and having two ends, molten metal supplying means for supplying molten aluminum bearing alloy into the casting space from one end side of the casting space, cooling means for cooling, through a medium of the endless belts, the molten aluminum bearing alloy supplied into the casting space, and water spraying means for spraying water onto the cast plate from both sides thereof to cool the same, the cast plate being continuously cast in the casting space and fed out of the other end side of the casting space by the travelling of the belts (as a sixth feature).
According to the above-described continuous casting apparatus, water is sprayed onto both sides of the cast plate fed out of the casting space. The cast plate is cooled rapidly to be solidified. Consequently, the crystal can be prevented from being coarsened.
In a seventh feature, the water spraying means sprays water onto a portion of the cast plate immediately after the portion has been fed out of said other end of the casting space.
In an eighth feature, the continuous casting apparatus further comprises a splashproof member provided for preventing splash of the water sprayed from the water spraying means onto the cast plate, the splashproof member being disposed to cover both sides of the cast plate fed out of the casting space. The endless belts come into contact with the molten metal such that its temperature is increased. Accordingly, when the water sprayed by the water spraying means splashes onto the endless belts, there is a possibility that the splashed water may explosively be vaporized. In the above-described construction, the splashproof member prevents the water sprayed by the spraying means from splashing on the endless belts. Consequently, the water can be prevented from being explosively vaporized.
In a ninth feature, the splashproof member has at least an end at the casting space side, the end being inclined so as to gradually come close to the cast plate toward the casting space side. When the splashproof member is thus inclined, the splashed water strikes against the splashproof member to be repelled in such a direction that it leaves away from the casting space. Consequently, the water can more reliably be prevented from splashing onto the endless belts.
In a tenth feature, the continuous casting apparatus further comprises air spraying means forming an air curtain located nearer to the casting space side than the water spraying means is, thereby preventing the water sprayed by the water spraying means from splashing toward each belt side.